This invention relates to two high performance printers of the type which may be utilized, for example, in connection with memory typewriters.
Printers of the serial impact type have heretofore utilized variable reluctance linear stepper motors. For example, Chai U.S. Pat. No. 3,867,676 discloses a variable reluctance linear stepper motor of the linear type which is intended for use in a printer application. Typically, such linear stepper motors are characterized by an undesirably low force-to-mass ratio. This low force-to-mass ratio may be the result of one of a number of factors.
One such factor is the overall weight or mass of the moving member itself. For example, the embodiment of FIG. 1 of Chai shows a substantial amount of magnetic material in the passive magnetic member 1 which substantially contributes to the overall mass of the moving member. Where the member 1 functions as a moving slider, the force-to-mass ratio is undesirably low. The same is true with respect to Fredrickson U.S. Pat. No. 3,292,065. In this regard, it will be noted that both the Chai and Fredrickson passive members have substantially greater length than the active stators. As a result, the large portion of the mass of the passive member which extends beyond the stator is of no consequence in generating usable force. Further, the passive members of both Fredrickson and Chai are relatively thick in cross-section even between the teeth or discontinuities in the passive members. Accordingly, a rather unlimited longitudinal flux path through the magnetic material of the passive member may be established for flux leakage which reduces the force-to-mass ratio for the slider of the motor. Apparently, neither Fredrickson nor Chai are at all concerned with maximizing the force-to-mass ratio by minimizing flux leakage. In this connection, it is noted that the motor of FIG. 9 of Chai which achieves the minimum longitudinal flux leakage as the result of discontinuities between the teeth of the slider which extend perpendicular or transverse to the direction of movement of the slider is not characterized by an optimized force-to-mass ratio. Rather, the force-to-mass ratio is a result of the single tooth per stator pole as well as a slider which always includes a substantial portion which extends beyond the stator structure and thereby produces no force.
Schreiber et al U.S. Pat. No. 3,162,796 also discloses a variable reluctance stepper motor of the linear type but demonstrates no interest in achieving a high force-to-mass ratio. In all of the Schreiber et al embodiments, there is a single tooth per stator pole and the slider extends beyond the stator structure so as to produce a small force-to-mass ratio. In almost all of the Schreiber et al embodiments, there is no stator structure on the interior of the cylindrical slider, and the minimum thickness of the slider is almost as great as the maximum thickness so as to permit the return of the flux longitudinally through the slider which necessarily reduces the force-to-mass ratio. The emdodiment of FIGS. 14 and 15 does disclose the use of an interior stator structure which permits a "reduction in weight" of the slider although there is no suggestion that the force-to-mass ratio is increased and the shape of the teeth which preclude any effective generation of force by the exterior stator structure in FIG. 14 and the interior stator structure of FIG. 14 suggests a low force-to-mass ratio. In connection with FIG. 15, there is the suggestion that the portion of the slider between the teeth may even comprise a non-magnetizable material. However, there is no suggestion that the non-magnetizable material is chosen for purposes of limiting longitudinal flux leakage and the suggestion that the material be "austen boron steel" precludes a further reduction in the force-to-mass ratio.
An article entitled Characteristics of a Synchronous Inductor Motor, Snowdon and Madsen, Trans. AIEE (applications in industry) Vol. 8, pp. 1-5, March, 1962, discloses a stepper motor having a rotor which is confined to the air gap of the stator. However, the force-to-mass ratio is relatively small since the rotor acts as a longitudinal flux return path to a single-sided stator. In order to provide this longitudinal flux return path, the minimum thickness of the rotor between the teeth of the rotor is substantial relative to the maximum thickness of the rotor at the teeth. An article entitled A Self-Oscillating Induction Motor for Shuttle Propulsion, Laithwaite and Lawrenson, Proc. IEE, Vol. 104, Part A, No. 14, April 1957, suggests that the rotor of Snowdon and Madsen might be unwound. However, the resulting slider would still have to provide a longitudinal flux return path for a single-sided stator. Therefore, even if the slider were shortened as disclosed in an article entitled Linear Induction Motors, Laithwaite, IEE, Paper No. 2433U, December 1957, the slider would still have a relatively low force-to-mass ratio and while this force-to-mass ratio might be increased by utilizing the double-sided stator disclosed in Linear Induction Motors, the configuration of the slider with its longitudinal flux leakage still severely limits the force-to-mass ratio.
Where the magnetic structure of the linear stepper motor results in a low force-to-mass ratio as described in the foregoing, the necessary force for high performance printing applications can only be achieved by driving the motor harder and this imposes requirements for larger power supplies. Where space, cost and heat are limiting factors in such printers, increased requirements for power supplies are undesirable.
In addition to the above-mentioned limitations on using linear stepper motors in high performance printing applications, there are also certain geometrical aspects which have heretofore created difficulties. In stepper motors which maximize the force-to-mass ratio by utilizing a stator which is energized by windings on both sides of a stator-to-stator air gap, certain difficulties are encountered in mounting the load on a slider which moves through the air gap. For example, where the print applying mechanism comprises a serial impact printing element such as a print wheel including circumferentially disposed character elements driven by a rotary motor, the print applying mechanism extends substantially above the slider and stator so as to result in an undesirably high center of gravity.